The performance levels of various semiconductor devices, such as transistors, are at least partly dependent on the mobility of charge carriers (e.g., electrons and/or electron vacancies, which are also referred to as holes) through the semiconductor device. In a transistor, the mobility of the charge carriers through the channel region is particularly important.
Various techniques have been used to improve charge carrier mobility in semiconductor devices. For example, a nitride layer can be formed on the source/drain regions of semiconductor device to induce horizontal tensile stress in the channel region of the device, which can improve charge carrier mobility in an n-type metal oxide semiconductor (“NMOS”) device. However, the amount of stress induced by the nitride layer is limited by the maximum intrinsic stress achievable by the nitride layer and the practical maximum thickness of the nitride layer.
Alternatively, a strained layer of silicon can be formed on a layer of relaxed silicon germanium in a channel region of an NMOS device. The term “strained” is used to describe a layer whose lattice structure of atoms is not typical for the material of which the layer is comprised. A layer of material (e.g., a first layer) can become strained when it is formed on a second layer of material with a different lattice structure (e.g., larger or smaller) than that of the first layer. A layer is “relaxed” when it has a lattice structure that is typical for the type of material of which the layer is comprised, in the absence of outside forces acting on the lattice.
The technique of forming strained silicon on relaxed silicon germanium (e.g., the silicon is strained by the larger size of the silicon germanium lattice) has the potential to induce a large amount of stress in the channel region of the NMOS device, which would yield large performance benefits. However, this technique requires formation of a large area of defect-free strained silicon, which is generally very difficult and expensive.
If the strained silicon layer has a high level of defects, the charge carrier mobility may be decreased through that layer of the device. For example, a dislocated charge carrier is a type of defect that may reduce charge carrier mobility by creating a local scatter area for the charge carriers, which can act as a leakage path that causes power loss through that section of the device.